Slicing an onion often causes stinging eyes and involuntary tears, making meal preparation a tearful experience for many. However, some individuals remain unaffected, seemingly immune to “onion tears.” This difference prompts a closer look into the science behind why certain people can chop onions without shedding a single tear.
The Chemical Culprit: How Onions Make Us Cry
Onions possess a natural defense mechanism. When their cells are damaged, such as during slicing, they release specific compounds. Enzymes called alliinases, normally kept separate within the onion’s cellular compartments, mix with sulfur-containing amino acids. This initial reaction forms unstable sulfenic acids.
A second enzyme, lachrymatory factor synthase (LFS), then acts upon these sulfenic acids. This enzymatic conversion produces syn-propanethial S-oxide, a volatile chemical compound referred to as the lachrymatory factor (LF). The LF quickly vaporizes and disperses into the air.
When this gaseous LF reaches the eyes, it irritates the sensory nerves on the cornea. The LF also reacts with water in the eyes’ tear film, forming a dilute solution of sulfuric acid. This acid causes a burning sensation, triggering reflex tears as a protective measure to flush out the irritant.
Individual Differences in Tear Response
Not everyone reacts with the same intensity to onion-induced tearing. Some individuals exhibit a reduced tear response, a variation linked to biological and physiological factors. Research indicates that a person’s ability to produce tears in response to the lachrymatory factor is influenced by their genetic makeup.
The ALDH1A1 gene plays a role in the metabolism and breakdown of onion LF. Individuals with a particular genetic variant of this gene tend to be less prone to tearing when cutting onions. Conversely, those without this variant may experience greater sensitivity to the lachrymatory factor. These differences in inherent sensitivity explain why some people are less affected by onion fumes.
External Factors Influencing Onion Irritation
Beyond individual biological variations, external factors and preparation techniques can reduce the tear-inducing effects of onions. Onion type and freshness play a role; varieties like yellow, red, and white onions contain higher concentrations of sulfur compounds and generally cause more irritation. Sweeter onion types, such as green onions, have less sulfur and are less pungent. Specialized “tear-free” onions, like Sunions, have been developed to produce lower levels of the lachrymatory factor.
The tools and methods used for cutting also influence the release of irritants. Employing a sharp knife minimizes cell damage, which reduces the amount of lachrymatory factor released into the air. A dull knife crushes more cells, leading to a greater release of tear-producing compounds. Additionally, avoiding the root end until the final cuts can help, as this part of the onion contains a higher concentration of the tear-inducing compounds.
Environmental conditions and strategic cutting practices can further mitigate the effects. Working in a well-ventilated area, such as near an open window or under an exhaust fan, helps disperse the volatile compounds away from the eyes. Chilling onions in the refrigerator or freezer before cutting can slow down the enzymatic reactions that produce the lachrymatory factor, reducing its release. Cutting onions under running water can also help wash away the irritants before they reach the eyes.